progress Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems D S Schimel1 21 J I House1 K A Hibbard2 P Bousquet3 P Ciais3 P Peylin4 B H Braswell13 M J Apps5 D Baker6 A Bondeau7 J Canadell8 G Churkina1 W Cramer7 A S Denning9 C B Field10 P Friedlingstein3 C Goodale10 M Heimann1 R A Houghton11 J M Melillo12 B Moore III13 D Murdiyarso14 I Noble15 S W Pacala16 I C Prentice1 M R Raupach17 P J Rayner18 R J Scholes19 W L Steffen20 C Wirth1 Knowledge of carbon exchange between the atmosphere land and the oceans is important given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil fuel combustion This carbon uptake is therefore limiting the extent of atmospheric and climatic change but its long term nature remains uncertain Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems Atmospheric carbon dioxide and oxygen data con rm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s but became a net carbon sink in the 1990s This recent sink can be largely attributed to northern extratropical areas and is roughly split between North America and Eurasia Tropical land areas however were approximately in balance with respect to carbon exchange implying a carbon sink that offset emissions due to tropical deforestation The evolution of the terrestrial carbon sink is largely the result of changes in land use over time such as regrowth on abandoned agricultural land and re prevention in addition to responses to environmental changes such as longer growing seasons and fertilization by carbon dioxide and nitrogen Nevertheless there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes A proportion of the carbon dioxide emitted to the atmosphere by fossil fuel burning and terrestrial processes mainly deforestation is taken up by the oceans and the terrestrial biosphere High precision atmospheric observations of concentrations of CO2 and O2 as O2 N2 ratios make it possible to partition the uptake of atmospheric CO2 between the land and ocean with increased con dence1 3 Global carbon budgets have been updated in the most recent IPCC assessment4 using this approach Table 1 The net terrestrial biospheric ux between the land and atmosphere was about 0 2 gigatonnes of carbon per year Gt C yr 1 in the 1980s and 1 4 Gt C yr 1 in the 1990s the negative sign denotes a ux from the atmosphere The net terrestrial biospheric ux is the difference between terrestrial uptake sinks and sources Estimates of land use change suggest emissions in the range of 0 6 to 2 5 Gt C yr 1 for the 1980s largely from deforestation in the tropics4 If emissions due to land use change were of a similar magnitude in the 1990s this would imply a residual terrestrial sink of between about 2 and 4 Gt C yr 1 Spatial patterns of carbon uptake Over the past two decades evidence has accumulated of signi cant contributions of extratropical Northern Hemisphere land areas to the global uptake of anthropogenic CO2 this evidence has been obtained from analysis of land inventory data5 8 atmospheric CO2 data9 13 atmospheric O2 data1 3 isotopic analyses3 14 studies of land use change15 and ecosystem process models16 17 Figure 1 shows zonal ux estimates from an ensemble of global atmospheric inverse model calculations this is a method that back calculates 1 Max Planck Institute fu r Biogeochemie Jena Germany 2 IGBP GAIM University of New Hampshire Morse Hall Durham New Hampshire 03824 USA 3 LSCE Unite mixte CEA CNRS Bat 709 CE L Orme des Merisiers 91191 Gif sur Yvette France 4 Laboratoire de Bioge ochimie Isotopique Unite mixte CNRSUPMC INRA 4 Place Jussieu 75252 Paris France 5 Natural Resources Canada Canadian Forest Service Northern Forestry Center 5320 122 Street Edmonton Alberta Canada 6 NCAR 1850 Table Mesa Drive Boulder 80303 USA 7 Potsdam Institute for Climate Impact Research Telegrafenberg C4 14473 Potsdam 8 GCTE International Project Of ce CSIRO Sustainable Ecosystems PO Box 284 Canberra ACT 2601 Australia 9 Department of Atmospheric Science Colorado State University Fort Collins Colorado 80523 1371 USA 10 Carnegie Institution of Washington Department of Plant Biology 260 Panama Street Stanford California 94305 USA 11 Woods Hole Research Center PO Box 296 Woods Hole Massachusetts 02543 USA 12 Ecosystems Center Marine Biological Laboratory Woods Hole NATURE VOL 414 8 NOVEMBER 2001 www nature com sources and sinks of CO2 from the distribution of atmospheric concentrations resulting in a range of estimates of the northern extratropical net land sink from 0 6 to 2 3 Gt C yr 1 in the 1980s13 Broad longitudinal partitioning of uxes remains less certain than broad latitudinal partitioning mainly because the former give rise to smaller differences in concentration14 18 Inverse model results are highly sensitive to the subset of atmospheric data used to the spatial and temporal resolution of the calculation method and to the atmospheric model used12 as indicated in Table 2 For example the estimates of the North American sink range from 0 to 88 of the total northern land sink depending on the approach Recent inverse modelling studies of the northern extratropical sink do not indicate a large imbalance of uxes between North Table 1 Contemporary carbon budgets for the 1980s and 1990s 1980s Gt C yr 1 1990s Gt C yr 1 5 4 6 0 3 6 3 6 0 4 3 3 6 0 1 1 9 6 0 5 0 2 6 0 7 1 7 0 6 to 2 5 1 9 3 8 to 0 3 3 2 6 0 1 1 7 6 0 5 1 4 6 0 7 Assume 1 6 6 0 8 2 to 4 Highly uncertain Emissions fossil fuel burning cement manufacture Atmospheric increase Ocean atmosphere ux Land atmosphere ux Emissions due to land use change Residual terrestrial sink Negative values denote ux from the atmosphere that is ocean or land uptake From ref 4 The range of estimates available to IPCC 2001 ref 4 Based on the early 1990s only and not the full decade ref 24 Massachusetts 02543 USA 13 University of New Hampshire Institute for the Study of Earth Oceans and Space Durham New Hampshire 03824 USA 14 GCTE Impacts Center for Southeast Asia Jalan Raya Tajur Km 6 POB 116 Bogor Indonesia 15 Ecosystem Dynamics RSBS Australian National University POB4 Acton Canberra ACT 0200 Australia 16 Department of Ecology and Evolutionary Biology Princeton University Princeton New Hampshire 08544 1003 USA 17